I think it's a personal choice which has benefits to many of the volunteers as well as to the people helped by the work the volunteers do.
There are plenty of older people who volunteer for the company of others, to be engaged in their community & to help.
There are also those who might look okay to you, but don't have good enough health to keep working in any capacity.
Nobody has a moral obligation to work for free, even if they retired at 40.
Also, no retired person has a moral obligation to be free childcare for their own adult children, so it's offensive to suggest that those grandchildren are their responsibility & some sort of 'excuse' for why they shouldn't be working for free for everyone else just because they didn't die yet.
Retired people who are out having lunch, visiting places, buying things with their money etc.. are SPENDING money which means that younger people have a job supplying the goods & services retired people are buying.
If they were all out volunteering, they wouldn't have time to spend their money & those paying jobs would disappear.
They aren't usually done by retired people.
I hate this belief that if you have a shit life, it's only fair everyone else does too.